In phase II testing of a new drug designed to increase the red blood cell count, a researcher obtains envelopes with the names and addresses of all treated subjects. She wants to increase the dosage in a sub-sample of 12 subjects, so she thoroughly mixes all of the envelopes in a bin, then pulls 12 of those envelopes to identify the subjects to be given the increased dosage.

Identify which of the following applies: simple random sample, systematic sample, convenience sample, stratified sample, or cluster sample. In each case, state whether you think the procedure is likely to yield a representative sample or a biased sample, and explain why.